there are many, many ways to increase the difficulty without creating specific forced combat roles like the traditional trinity.
Your Challenger won't drive? You're probably in the passenger seat.
Can't sell/delete an item? Check all of your loadouts, it's likely still equipped.
Auction house? Why? It'll just render bits purchased lock boxes pointless, and weapons are mostly equal anyway.
Gear/enemy progression? No thanks, it'll only render 90% of the content trivial when you reach the highest zone.
I can agree with making co-op maps that are more complex and/or difficult, but I can not agree with "forcing holy trinity" on to a player base.
I am all for making fight more complex/difficult by adding things such different objectives, or by making more use of different grenade and nano effects.
The tank/healer/dps set-up is limiting and placing it in a system that is for all intents and purposes limitless, will hurt more than it helps. As what happens with most trinity games, alienation, burnout and long wait periods will set in. I can see it now, "LFHealer ER 1500+ BMG lvl 15+ 101 keys hard mode must have 4/4 mod BMG, no stingrays plz". Or "LF Tank ER 3000+ for Lost Soldiers LMG with biological nano effect only." The tanks and healer get blamed for wipes and burnout. The DPS get to wait in long ques b/c the tanks and healers will do guild runs only. The same story plays out same way almost every time in a mmo with a trinity system for pve.
I think adding enemy patrols maybe enemies that can detect cloaked players if they're in their sight too long and suppressors as a barrel mod would be a good idea . Have it decrease the radius that enemies will react to your gun fire but lower your falloff damage, with the tweaks in difficulty it would add a bit more variety to peoples play styles to compliment their perks without really grouping anyone into a specific role or requiring a trinity. Just another way to play. The added tweaks to difficulty would likely make the bmg more useful then just add a leaderboards at the end that show scoring for something other than damage done